Process for preparing high molecular weight polyoxymethylenes



United States Patent 3,415,783 PROCESS FOR PREPARING HIGH MOLECULAR WEIGHT POLYOXYMETHYLENES Silvio Bezzi and Luigi Mortillaro, Padova, and Alberto Bandel, Venice, Italy, assignors to Montecatini Edison S.p.A., Milan, Italy No Drawing. Filed Jan. 30, 1964, Ser. No. 342,334 Claims priority, application Italy, Jan. 31, 1963,

1,929/ 63 t 14 Claims. (Cl. 260-67) This invention is directed to a process for preparing high molecular weight polyoxymethylenes and more specificallyis directed to a method of preparing polyoxymethylenes from aqueous solutions of formaldehyde. Still more specifically, this invention is directed to a method of preparing polyoxymethylenes 'by reacting an aqueous solution of formaldehyde with a small amount of pro-formed polyoxymethylene in the presence ofat least one salt of an inorganic acid as the catalyst. The high molecular weight polyoxymethylenes of this invention have a density in excess of 1.5 g. per cc. at C. and a crystalline-orthorhombic structure.

It is disclosed in copending application Ser. No. 168,220, filed on Jan. 23, 1962, that the polymerization of aqueous solutions of formaldehyde can be'catalyzed with either a quaternary amine or a metal salt of an organic acid. It has now been discovered, however, that in addition to the organic acid salts, inorganic salts can be used, quite successfully, as a catalyst for the polymerization of formaldehyde. The catalytic action during the polymerization will vary depending upon the nature of the anions and cations of the salts. More specifically, these inorganic acid salts are salts of metals of Groups I and II of Mendelefis Periodic Table and may be used as a mixture with each other and/ or with other organic acid salts of the metals of Groups I and II of the Periodic Table. In addition, it is possible to use the quaternary amines either with at least one of these inorganic acid salts or with a combination of the inorganic acid salts and the organic acid salts. The inorganic acid salts coming within the scope of this invention include, for example, potassium fluoride, lithium chloride, sodium chloride, potassium chloride, calcium chloride, sodium nitrate, etc.

It has been found that by using these inorganic acid salts as catalysts it is possible to obtain at relative low temperatures, e.g., below 60 C. uniformity with regard to boththe amount of polymer produced each hour and its average molecular weight. These uniform conditions remained constant over an indefinite period of time. It has been discovered, quite unexpectedly, that in comparison to using the organic acid salts, e.g. sodium formate, the inorganic salts of this invention, e.g., potassium fluoride, make it possible to obtain under the same operative conditions, either polyoxymethylenes which have a much' higher molecular weight or else products of the same molecular weight but with higher production kinetics.

Accordingly, it is an object of this invention to pro vide a process for preparing high molecular weight polyoxymethylene having a density in excess of 1.5 g. per cc. at 20 C. and a crystalline-orthorhombic structure. These polymers are prepared from aqueous solutions of formaldehyde having a concentration ranging from about the point of equilibrium to the point of saturation. Above the saturation point, the solutions are unstable and spontaneous separation of the polyoxymethylene will take place. These aqueous solutions of formaldehyde are reacted with a small amount of a solid phase of preformed polyoxy- "ice methylene at a pH above 7 and more preferably at a pH between 9.3 and 12. The temperature of the reaction medium ranges from about 0 C. and'60 C. and more preferably between 20 C. and 40 C. The concentration of formaldehyde in the aqueous medium and the pH value is held constant by periodically adding more formaldehyde and alkaline reagents. The reaction is particularly characterized in that the polymerization is carried out in the presence of a catalytic amount of at least one, or more salts of an inorganic acid wherein the metal of the salt is selected from either Group I or II of Mendelefis Periodic Table. The concentration of the inorganic catalyst in the aqueous medium is held constant and is used in an amount ranging from about 6% by weight of the solution to the point of saturation. Likewise, this concentration is the same whether the salt is used alone, or in combination with one or more of the organic acid salts of the metals of Groups I and II of the Periodic Table.

More specifically, the polyoxymethylenes of this invention are prepared by suspending a small but initiating amount 'of polyoxymethylene in an aqueous solution containing formaldehyde at a concentration above the point of equilibrium. This aqueous reaction medium is held at a temperature below 60 C. and at a. pH above 7 and contains a catalytic proportion of at least one of the above-mentioned inorganic acid salts in an amount ranging from a'boutat least 6% by weight of the solution. A saturated solution of formaldehyde is added continuously or in short intervals to the suspension in order to restore the amount of formaldehyde lost due to polymerization and to compensate for the losses caused by a Cannizzaro reaction. Likewise, an alkaline reagent, e.g., KOI-I or NaOH and at least one of the above-mentioned inorganic acid salts may be continuously added in order to maintain a uniform composition and to keep the solidto-liquid ratio and the pH constant over a period of time.

In each of the following examples, samples of the polymer were periodically withdrawn from the reaction medium so as to determine, through viscosity measurements, the variation in the molecular weight and the degree of c-rystallinity or kind of structure. Thus, it has been found that polyoxymethylenes can be prepared which are essentially crystalline and have an orthorhombic structure or form. The degree of crystallinity will increase by lowering the reaction temperature. In making the character determinations, each sample was washed in water in an amount twice its weight and then washed in an equal amount of benzene. The samples were dried for about 12 hours at 40 C. under a pressure of 15 mm. and then subjected to acetylation by heating the sample in a closed vial with acetic anhydride at a temperature of 170 C. The acetic anhydride was free from acetic acid and was present in an amount correspondnig to about 10 times the weight of the polymeric sample. The vial was immersed in an oil bath at a temperatrue of 170 C. and held in the bath, with agitation, for 3 to 5 minutes until the polyoxymethylene was dissolved. By following the method specifically described by Staudinger (Ann. 474; 174-175 (1929)), a yield of 90% of acetylated product was obtained.

The reduced voscosity values, as reported in the following tables, were measured in dimethylformamide at C. at a concentration of 0.5% (1 red. 0.5). The reduced viscosity is defined as:

n spec. 1; rel- 3 wherein equals the concentration in g./ 100 cc.

Any variation in the concentration of the inorganic salts, or in the pH of the solution, or in the kinetics has a corresponding influence on the molecular complexity of the polymer. Thus, with respect to the concentration of the salts, it has been found that as the concentration increases the molecular weight of the polyoxymethylene also increases, see Table 1. Likewise, as the salt concentration increases there is an increase in the kinetics at which the polymerization can be carried out, see Table 2. With respect to pH, it has been found that it is preferred to operate at pH values ranging from about 9.3 to 12.0. In this range there is an optimum pH value which makes it possible to obtain higher molecular weights with the same kinetics (see Tables 3, 4, and or to obtain polyoxymethylenes with the same molecular weights but at higher kinetics, see Table 6. The kinetics are expresed as grams per hour for every hundred grams of the pre-formed solid polymer present. The molecular complexity of the polymer was determined by measuring the viscosity at 150 C. in dimethylformamide.

The following tables and accompanying examples are given to give a more complete and specific illustration of the invention. The pH values reported in each example were measured with a glass electrode having an expanded scale, as produced by the Beckman Co.

TABLE 1 T=35 C.; K=1.4; pH=lO.3 to 10.0.

KF concentration 7; red. of the percent by weight polymer obtained 9. 5 wqcam moo-o The above table illustrates the influence of the salt concentration on the molecular complexity of the polyoxymethylene.

The above table illustrates the influence of the salt concentration and of the production kinetics on the molecular 5 complexity of the polyoxymethylenes.

TABLE 3 T= 0.; K=3.0; KF=32.4 by Weight.

1; red. of the polymer obtained The above table illustrates the influence of pH on the molecular complexity of polyoxymethylene.

1 red. of the polymer obtained Example: 10

ocoqw mo The above table illustrates the influence of the pH on the molecular complexity of polyoxymethylene.

TABLE 5 T=35 0.; K 1.4; KF=l525 by Weight.

pH 1; red. of the polymer obtained Example:

The above table illustrates the influence of pH on the molecular complexity of polyoxymethylene.

TABLE 6 T=35 C.: KF=32A to 32.7 by weight.

Kinetics pH 1; red. of the polymer obtained Example:

The above table illustrates the influence of pH and of the production kinetics on the molecular complexity of polyoxymethylene.

EXAMPLE 1 Grams 50% by Weight of formaldehyde solution 27.8 10 N NaOH solution 0.3

The amount of NaOH was sufficient to hold the pH of the suspension at about 10.4 for the entire reaction period.

After every '24 hours, about 337 g. of the suspension were discharged. After the discharged product was filtered, washed and dried, about 118 g. of polyoxymethylene were obtained, which corresponds to about 0.7 g. per hour per g. of preformed solid polymer. The reaction was stopped after 32 days with the amount of polymer obtained each day being substantially the same. The variation in the characteristics of the polymer, with respect to time, are reported in Table 7.

TABLE 7 T 20;l; 1; pH =10.4:l;0.2; Kineties=0.7.

Characteristics of the Polymer The above table illustrates the variation with respect to time in the characteristics of the polyoxymethylene as obtained in accordance with Example 1. It should be noted that the molecular complexity of the polymer reached a stationary condition after about 4 days. The molecular weight of these polyoxymethylenes were low and were prepared with the use of HCOONa concentration of about 0.9%.

EXAMPLE 2 About 2 kg. of a suspension consisting of 693 g. of polyoxymethylene and 1307 g. of an aqueous solution containing 13% lithium chloride and 15% of formaldehyde were introduced into a 2-liter reactor provided with an agitator and immersed in a thermostatic bath at 20 C. The starting polyoxymethylene was characterized as having an 1 red. value of 0.77 and a crystalline orthorhombic form, as determined by X-ray examination, of 95%.

To this suspension the following reactants were added after having removed an equivalent amount from the reactor:

Grams 50% by weight CH O solution 24.9 Lithium chloride powder 2.25 LiOH solution 1.0

The amount of lithium hydroxide was sufiicient to hold the pH of the suspension at about 10.4 for the entire reaction period.

At every 24 hours, about 337.8 g. of the suspension were discharged. After the discharged product was filtered, washed and dried, about 97.5 g. of polyoxymethylene were obtained, which corresponds to about 0.7 g. per hour per 100 g. of preformed solid polymer. The reaction was stopped after days with the amount of polymer obtained each day being substantially the same. The variation in the characteristics of the polymer, with respect to time, are reported in Table 8. The molecular complexity remained substantially unchanged after a period of 3 days.

TABLE 8 T= Oil; pH=10.4=|;0.2; Kinetics=0.7.

Characteristics of the Polymer 1, red. Crystalline, Orthorhombic torn; (percent) The above table illustrates the variation with respect to time in the characteristics of the polyoxymethylene as obtained in accordance with Example 2.

EXAMPLE 3 Grams 50% by weight of CH O solution 24.1 Powdered potassium chloride 3.84 40% KOH solution 0.3

The amount of KOI-I was sutficient to hold the pH of the suspension at about 10.4 for the entire reaction period.

After every 24 hours, about 339 g. of the suspension were discharged. After the discharged product was filtered, Washed and dried, about 109 g. of polyoxymethylene were obtained, which corresponds to about 0.7 g. per hour per 100 g. of preformed solid polymer. The reaction was stopped after 28 days with the amount of polymer obtained each day being substantially the same. The variation in the characteristics of the polymer with respect to time are reported in Table 9. The molecular complexity remained substantially unchanged from the beginning of the reaction.

TABLE 9 T=20 C.=l;1; pH=10.4;l=0.2; Kinetics=0.7.

Characteristics of the Polymer 1; red. Crystalline, Orthorhombic form (percent) Days Elapsed:

The above table illustrates the variation with respect to time in the characteristics of the polyoxymethylene as obtained in accordance with Example 3.

EXAMPLE 4 actor:

Grams 50% by weight of CI-I O solution 24.8 Powdered sodium chloride 3.1 NaOH solution 10 N 0.25

The amount of sodium hydroxide was suflicient to hold the pH of the suspension at about 10.4 for the entire reaction period.

After every 24 hours, about 237 g. of the suspension were discharged. After the discharged product was filtered, washed and dried, about 116.5 g. of polyoxymethylene were obtained, which cor-responds to about 0.7 g. per hour per 100 g. of preformed solid polymer. The reaction was stopped after 17 days with the amount of polymer obtained each day being substantially the same. The variation in the characteristics of the polymer, with respect to time, are reported in Table 10. The molecular complexity remained unchanged after a period of 8 days.

TABLE 10 T=20 0.5:1; pH=10.4i0.2; Kinetics=0.7.

Characteristics of the Polymer 1 red. Crystalline, Orthorhombie forrn (percent) Da s Elapsed:

The above table illustrates the variation with respect to time in the characteristics of the polyoxymethylene as obtained in accordance with Example 4.

EXAMPLE 5 after having removed an equivalent amount from the reactor:

Grams 50% by weight of CH O solution 24.37 Powdered calcium chloride 2.88 10 N NaOH solution 0.7

The amount of sodium hydroxide was sufficient to hold the pH of the suspension at about 10.2 for the entire reaction period.

After every 24 hours, about 335 g. of the suspension were discharged. After the discharged product was filtered, washed and dried, about 112 g. of polyoxymethylene, which corresponds to about 0.7 g. per hour per 100 g. of preformed solid polymer, were obtained. The reaction was stopped after 28 days with the amount of polymer obtained each day being substantially the same. The variation in the characteristics of the polymer with respect to time are reported in Table 11. The molecular complexity remained substantially unchanged after a period of 10 days.

TABLE 11 T= (15:1; pH=10.2;i;0.2; Kinetics=0.7.

Characteristics of the Polymer 1, red. Crystalline, Orthorhombic form (percent) Days Elapsed:

The above table illustrates the variation with respect to time in the characteristics of the polyoxymethylene as obtained in accordance with Example 5.

EXAMPLE 6 reactor:

Grams 50% by weight of CH O solution 23.3 Powdered sodium nitrate 4.3 10 N NaOH solution 0.4

The amount of sodium hydroxide was sufficient to hold the pH of the suspension at about 10.4 for the entire reaction period.

After every 24 hours, about 337 g. of the suspension were discharged. After the discharged product was filtered, washed, and dried, about 106 g. of polyoxymethylene were obtained, which corresponds to about 1.4 g. per hour per 100 g. of preformed solid polymer. The reaction Was stopped after 32 days with the amount of polymer obtained each day being substantially the same. The variation in the characteristics of the polymer, with respect to time, are reported in Table 12. The molecular complexity remained substantially unchanged after a period of 18 days.

TABLE 12 T=20 Gil; pH+l0.4;|;0.2; Kinetics-=07.

Characteristics of the Polymer 11 red. Crystalline, Orthorhombic form (percent) Da s Ela sed:

The above table illustrates the variation with respect to time in the characteristics of the polyoxymethylene as obtained in accordance with Example 6.

EXAMPLE 7 Grams 50% by Weight of CH O solution 24.4 Powdered potassium chloride 1.93 Powdered sodium. chloride 1.51 40% NaOH solution 0.25

The NaOH amount was sufficient to hold the pH of the suspension at about 10.4 for the entire reaction period.

After every 24 hours, about 337 g. of the suspension were discharged. After the discharged product was filtered, washed, and dried, about 112 g. of polyoxymethylene were obtained, which corresponds to about 0.7 g. per hour per 100 g. of preformed solid polymer. The reaction was stopped after 28 days with the amount of polymer obtained each day being substantially the same. The variation in the characteristics of the polymer, with respect to time, are reported in Table 13. The molecular complexity remained substantially unchanged after a period of 18 days.

TABLE 13 T=20 Oil; pI-I= 10.4102; Kinetics=0.7.

Characteristics of the Polymer 1, red. Crystalline, Orthorhombic form (percent) Dayos Elapsed:

The above illustrates the variation with respect to time in the characteristics of the polyoxymethylene as obtained in accordance with Example 7.

EXAMPLE 8 To this suspension the following reactants were added after having removed an equivalent amount from the reactor:

Grams 50% by weight of CH O solution 28.0 10 N NaOH solution 1.2

The amount of sodium hydroxide was suflicient to hold the pH of the suspension at about 10.4 for the entire reaction period.

After every 24 hours, about 686 g. of the sunpension were discharged. After the discharged product was filtered, washed, and dried, about 188 g. of polyoxymethylene were obtained, which corresponds to about 1.4 g. per hour per 100 g. of preformed solid polymer. The reaction was stopped after 31 days with the amount of polymer obtained each day being substantially the same. The variation in the characteristics of the polymer, with respect to time, are reported in Table 14. The molecular complexity remained substantially unchanged after a period of 14 days.

TABLE 14 T=35 C.:l=1; pH=10.4=l=0.2; Kinetics=1.4.

Characteristics of the Polymer 1 red. Crystalline, Orthorhombic form (percent) Dayg Elapsed:

: 9. 5 5 9?? rommmmwceeehen Qmooqwowwwu:

EXAMPLE 9 Grams 50% by weight of CH O solution 20 Powdered sodium nitrate 8.5 10 N NaOH solution 0.8

The amount of NaOH was sufficient to hold the pH of the suspension at about 10.4 for the entire reaction period.

After every 24 hours, about 703 g. of the suspension were discharged. After the discharged product was filtered, washed, and dried, about 174 g. of polyoxymethylene were obtained, which corresponds to about 1.4 g. per hour per 100 g. of preformed solid polymer. The reaction was stopped after 42 days with the amount of polymer obtained each day being substantially the same. The variation in the characteristics of the polymer, with respect to time are reported in Table 15. The molecular complexity remained substantially unchanged after a period of 11 days.

7 TABLE 15 'r=35 can; pH=l0.4=t:0.2; Kinetics=l.4.

Characteristics of the Polymer 1; red. Crystalline, Orthorhombic form (percent) Days Elapsed:

The above table illustrates the variation with respect to time in the characteristics of the polyoxymethylene as obtained in accordance with Example 9.

EXAMPLE 10 About 2 kg. of a suspension consisting of 511 g. of polyoxymethylene and 1489 g. of an aqueous solution containing 32.7% potassium fluoride and 15.5% of formaldehyde were introduced into a 2-liter reactor provided with an agitator and immersed in a thermostatic bath at 35 C. The starting polyoxymethylene was characterized as having an 1 red. value of 0.91 and a crystalline orthorhombic form, as determined by X-ray examination, of 64%.

To this suspension the following reactants were added after having removed an equivalent amount from the reactor:

Grams 50% by weight CH O solution 20.9 Powdered potassium fluoride 6.6

The pH of the suspension was held at about 9.3 for the entire reaction period.

After every 24 hours, about 684 g. of the suspension were discharged. After the discharged product was filtered, Washed and dried, about 175 g. of polyoxymethylene were obtained, which corresponds to about 1.4 g. per hour per g. of preformed solid polymer. The reaction was stopped after 20 days with the amount of polymer obtained each day being substantially the same. The variation in the characteristics of the polymer, with respect to time, are reported in Table 16.

TABLE 16 T=35 C.:t=l; pH=9.3i0.2; Kinetics=1.4.

Characteristics of the Polymer Crystalline, Orthorhombic form (percent) 1 red.

Days Elapsed;

The above table illustrates the variation with respect to time in the characteristics of the polyoxymethylene as obtained in accordance with Example 10.

EXAMPLE 11 Grams 50% by weight of CH O solution 21.5 Powdered potassium fluoride 6.8 40% KOH solution 0.07

The amount of KOH was suflicient to hold the pH of the suspension at about 9.9 for the entire reaction period.

After every 24 hours, about 680 g. of the suspension were discharged. After the discharged product was filtered,

washed, and dried, about 178 g. of polyoxymethylene were obtained, which corresponds to about 1.4 g. per hour per 100 g. of preformed solid polymer. The reaction was stopped after 24 days with the amount of polymer obtained each day being substantially the same. The variation in the characteristics of the polymer, with respect to time, are reported in Table 17. The molecular complexity 11 remained substantially unchanged after a period of 14 days.

TABLE 17 T=35 (Lil; pH=9.9:|;0.2; Kinetlcs=1.4.

Characteristics of the Polymer 1, red. Crystalline, Orthorhombic form (percent) Days Elapsed:

The above table illustrates the variation with respect to time in the characteristics of the polyoxymethylene as obtained in accordance with Example 11.

EXAMPLE 12 To this suspension the following reactants were added after having removed an equivalent amount from the reactor:

Grams 50% by weight of CH O solution 20.9 Powdered potassium fluoride 6.6 40% KOH solution 0.26

The amount of KOH was sufficient to hold the pH of the suspension at about 10.3 for the entire reaction period.

After every 24 hours, about 684 g. of the suspension were discharged. After the discharged product was filtered, washed, and dried, about 175 g. of polyoxymethylene were obtained, which corresponds to about 1.4 g. per hour per 100 g. of preformed solid polymer. The reaction was stopped after days with the amount of polymer obtained each day being substantially the same. The variation in the characteristics of the polymer, with respect to time, are reported in Table 18. The molecular complexity remained substantially unchanged after a period of 10 days.

TABLE 18 T= 0.11:1; pH=10.3i0.2; Kinetics=l.4.

Characteristics of the Polymer Crystalline, Ortho- 1; red.

rhombic form (percent) Dayos Elapsed:

EXAMPLE 13 after having removed an equivalent amount from the reactor:

Grams 50% by weight of CH solution 20.9 Powdered potassium fluoride 6.6 40% KOH solution 0.8

The amount of KOH was sulficient to hold the pH of the suspension at about 10.9 for the entire reaction period.

After every 24 hours, about 684 g. of the suspension were discharged. After the discharged product was filtered, washed and dried, about 175 g. of polyoxymethylene were obtained, which corresponds to about 1.4 g. per hour per g. of preformed solid polymer. The reaction was stopped after 28 days with the amount of polymer obtained each day being substantially the same. The variation in the characteristics of the polymer, with respect to time, are reported in Table 19. The molecular complexity remained substantially unchanged after a period of 14 days.

TABLE 19 T=35 C.:l:1; pl-l=10.9:b0.2; Kinetics=1.4.

Characteristics of the Polymer Crystalline, Ortho- Days Elapsed:

The above table illustrates the variation with respect to time in the characteristics of the polyoxymethylene as obtained in accordance with Example 13.

EXAMPLE 14 About 2 kg. of a suspension consisting of 487 g. of polyoxymethylene and 1513 g. of an aqueous solution containing 32.4% potassium fluoride and 16.5% of formaldehyde were introduced into a 2-liter reactor provided with an agitator and immersed in a thermostatic bath at 35 C. The searting polyoxymethylene was characterized as having an 1; red. value of 0.53 and a crystalline orthorhombic form, as determined by X-ray examination, of 74%.

To this suspension the following reactants Were added after having removed an equivalent amount from the reactor:

Grams 50% by weight of CH O solution 44.5 Powdered potassium fluoride 14.0 40% KOH solution 2.1

The amount of KOH was suflicient to hold the pH of the suspension at about 11.4 for the entire reaction period.

After every 24 hours, about 1454 g. of the suspension were discharged. After the discharged product was filtered, washed, and dried, about 353 g. of polyoxymethylene were obtained, which corresponds to 3.0 per hour per 100 g. of preformed solid polymer. The reaction was stopped after 24 days with the amount of polymer obtained each day being substantially the same. The variation in the characteristics of the polymer, with respect to time, are reported in Table 20. The molecular com- Characteristics of the Polymer 1 red. Crystalline, Orthorhombic form (percent) Days Elapsed:

The above table illustrates the variation with respect to time in the characteristics of the polyoxymethylene as obtained in accordance with Example 14.

EXAMPLE About 2 kg. of a suspension consisting of 487 g. of polyoxymethylene and 1513 g. of an aqueous solution containing 32.4% potassium fluoride and 16.5% of formaldehyde were introduced into a 2-liter reactor provided with an agitator and immersed in a thermostatic bath at 35 C. The starting polyoxymethylene was characterized as having an 7; red. value of 0.91 and a crystalline orthorhombic form, as determined by X-ray examination, of 64%.

To this suspension the following reactants were added after having removed the equivalent amount from the reactor:

Grams 50% by weight of CH O solution 44.5 Powered potassium fluoride 14.5 40% KOH solution 1.4

The amount of KOH was suflicient to hold the pH of the suspension at about 10 .9 for the entire reaction period.

After every 24 hours, about 1454 g. of the suspension were discharged. After the discharged product was filtered, washed and dried, about 535 g. of polyoxymethyions were obtained, which corresponds to about 3.0 g. per hour per 100 g. of preformed solid polymer. The reaction was stopped after 25 days with the amount of polymer obtained each day being substantially the same. The variation in the characteristics of the polymer, with respect to time, are reported in Table 21. The molecular complexity remained substantially unchanged after a period of 7 days.

TABLE 21 T=35 C.:l:1; pH=10.9d=0.2: Kinetics=3.

Characteristics of the Polymer Crystalline, Orthorhombic form (percent) 17 red.

Dayos Elapsed:

The above table illustrates the variation with respect to time in the characteristics of the polyoxymethylene as obtained in accordance with Example 15.

EXAMPLE 16 Grams 50% by weight of CH O solution 44.5 Powdered potassium fluoride 14.0 40% KOH solution 1.4

The amount of KO'H was sufficient to hold the pH of the suspension at about 10.9 for the entire reaction period.

After every 24 hours, about 1450 g. of the suspension were discharged. After the discharged product was filtered, washed, and dried, about 353 g. of polyoxymethylene were obtained, which corresponds to about 3 gl. per hour per 100 g. of preformed solid polymer. The reaction was stopped after 25 days with the amount of polymer obtained each day being substantially the same. The variation in the characteristics of the polymer, with respect to time, are reported in Table 22. The molecular complexity remained substantially unchanged after a period of 14 days.

TABLE 22 T=35 Oil; pH=10.9i0.2; Kineties=3.

Characteristics of the Polymer Crystalline, 1 red. Orthorhombie form percent Days Elapsed:

The above table illustrates the variation with respect to time in the characteristics of the polyoxyrnethylene as obtained in accordance with Example 16.

EXAMPLE 17 Grams 50% by weight of CH O solution 44.5 Powdered potassium fluoride 14.0 40% KOH solution 0.06

The amount of KOH was suificient to hold the pH of the suspension at about 9.9 for the entire reaction period.

After every 24 hours, about 1450 g. of the suspension were discharged. After the discharged product was filtered, washed and dried, about 353 g. of polyoxymethylene were obtained, which corresponds to about 3 g. per hour per g. of preformed solid polymer. The reaction was stopped after 24 days with the amount of polymer obtained each day being substantially the same. The variation in the characteristics of the polymer, with respect to time, are reported in Table 23. The molecular complexity remained substantially unchanged after a period of 7 days.

Crystalline, 1 red. orthorhombic tonn percent Days Elapsed:

The above table illustrates the variation with respect to time in the characteristics of the polyoxymethylene as obtained in accordance with Example 17.

EXAMPLE 18 Grams 50% by weight of (H 0 solution 24.5 Powdered potassium fluoride 3.0 40% KOH solution 0.4

The amount of KOH was sufficient to hold the pH of the suspension at about 9.9 for the entire reaction period.

After every 24 hours, about 672 g. of the suspension were discharged. After the discharged product was filtered, washed and dried, about 182 g. of polyoxymethylene were obtained, which corresponds to about 1.4 g. per hour per 100 g. of preformed solid polymer. The

.reaction was stopped after 24 days with the amount of polymer obtained each day being substantially the same. The variation in the characteristics of the polymer, with respect to time, are reported in Table 24. The molecular complexity remained substantially unchanged after a period of 14 days.

TABLE 24 T=35 C.=l=1; pH=9.9:l:0.2; Kinetics=1.4.

Characteristics of the Polymer Crystalline, n red. orthorhombic form percent Days Elapsed:

The above table illustrates the variation with respect to time in the characteristics of the polyoxymethylene as obtained in accordance with Example 18.

EXAMPLE 19 About 2 kg. of a suspension consisting of 544 g. of polyoxymethylene and 1456 g. of an aqueous solution containing 15.25% potassium fluoride and 23.0% of formaldehyde were introduced into a 2-liter reactor provided with an agitator and immersed in a thermostatic bath at 35 C. The starting polyoxymethylene was characterized as having an a red. value of 0.43 and a crystal- 16 line orthorhombic form, as determined by X-ray examination, of 90%.

To this suspension the following reactants were added after having removed an equivalent amount from the reactor.

Grams 50% by weight of CH O solution 24.5 Powdered potassium fluoride 3.0 KOH solution 0.4

The amount of KOH was sufficient to hold the pH of the suspension at about 10.6 for the entire reaction period.

After every 24 hours, about 672 g. of the suspension were discharged. After the discharged product was filtered, washed and dried, about 1 83 g. of polyoxymethylene were obtained, which corresponds to about 1.4 g. per hour per 10 0 g. of preformed solid polymer. The reaction was stopped after 25 days with the amount of polymer obtained each day being substantially the same. The variation in the characteristics of the polymer, with respect to time, are reported in Table 25. The molecular complexity remained substantially unchanged after a period of 11 days.

Characteristics of the Polymer Crystalline,

1 red. orthorhombic form percent Days Elapsed:

The above table illustrates the variation with respect to time in the characteristics of the polyoxymethylene as obtained in accordance with Example 19.

EXAMPLE 20 after having removed an equivalent amount from the. reactor:

Grams 50% by weight CH O solution 18.4 Powdered potassium fluoride 0.8 40% KOH solution 1.16

The amount of KOH was sufiicient to hold the pH of the suspension at about 10.4 for the entire reaction period.

After every 24 hours, about 490 g. of the suspension were discharged. After the discharged product was filtered, washed and dried, about 173 g. of polyoxymethylene were obtained, which corresponds to about 1. 4 g. per hour per .100 g. of preformed solid polymer. The reaction was stopped after 30 days with the amount of polymer obtained each day being substantially the same. The variation in the characteristics of the polymer, with respect to time, are reported in Table 2-6. The molecular complexity remained substantially unchanged after a period of 13 days.

TABLE 26 T=35 0.; pH=10.4:l;0.2; Kinetics=1.4.

Characteristics of the Polymer Crystalline,

orthorhombic form percent 1; red.

Dayos Elapsed:

999. 9. 99. 9 cemceeecewmmmq wo-u-uawmoqoo The above table illustrates the variation with respect to time in the characteristics of the polyoxymethylene as obtained in accordance with Example 20.

EXAMPLE 21 Grams 50% by weight of CH O solution 23.8 Powdered potassium formate 2.3 Powdered potassium fluoride 1.6 40% KOH solution 0.33

The amount of KOH was sufficient to hold the pH of the suspension at about 10.6 for the entire reaction period. After every 24 hours, about 670 g. of the suspension were discharged. After the discharged product was fil tered, Washed and dried, about 170 g. of polyoxymethylene were obtained, which corresponds to about 1.4 g. per hour per 100 g. of preformed solid polymer. The reaction was stopped after 32 days with the amount of polymer obtained each day being substantially the same. The variation in the characteristics of the polymer, with respect to time, are reported in Table 27. The molecular complexity remained substantially unchanged after a period of 11 days.

TABLE 27 T=35 0.; pH=10.6;l:0.2; Kinetics=1.4.

' Characteristics of the Polymer Crystalline, 1; red. orthorhombic form percent Ela sed:

oinf 0.43 90 0. 51 79 0. 65 81 0. 73 84 0. 72 85 0. 68 83 0. 70 84 0. 71 86 What is claimed is:

1. A process for synthesizing "high molecular weight polyoxymethylenes.nhaving an orthorhombic-crystalline structure which comprises polymerizing said polyoxy methylenes from ahTaqueous solution of formaldehyde by topochemical reaction on an effective amount of polyoxymethylene as the solid phase at a temperature ranging from about 0? .C. to C.; said aqueous solution of formaldehyde containing at [least one inorganic acid metal salt wherein the metal of the salt is selected from Groups I and II of Mendeleffs Periodic Table, said inorganic salt being present in the aqueous solution at a concentration of at least 6% by weight; said aqueous solution having a pH between 9 and 12 and a formaldehyde concentration ranging from the equilibrium concentration to the saturation concentration; said polymerization taking place while maintaining the pH and the con- ;centration of formaldehyde and inorganic acid salt substantially constant.

2. The process of claim 1 wherein the inorganic acid salt is present in an amount ranging from about 6% by weight of the solution to the point of saturation.

3. The process of claim 1 wherein the polymerization takes place at a temperature ranging from about 20 C. to 40 C.

4. The process of claim 1 wherein said concentrations of formaldehyde and inorganic acid salt are maintained substantially constant by periodically adding the respective reactants to the reaction.

5. The process of claim 1 wherein a saturated aqueous solution of formaldehyde and an inorganic acid salt are continuously added to the polymerization reaction while simultaneously discharging a dilute solution of formaldehyde and polymer.

6. The process of claim 1 wherein the aqueous solution of formaldehyde further contains at least one organic acid salt of a metal belonging to Groups I and II of the Periodic Table.

7. The process of claim 1 wherein the inorganic acid metal salt comprises sodium chloride.

8. The process of claim 1 wherein the inorganic acid metal salt comprises calcium chloride.

9. The process of claim 1 wherein the inorganic acid metal salt comprises sodium nitrate.

10. The process of claim 1 wherein the inorganic" metal salt comprises potassium fluoride. f

11. The process of claim 1 wherein the inorganic a'ci d metal salt comprises potassium chloride.

12. The process of claim 1 wherein the inorganic metal salt comprises lithium chloride.

13. The process of claim 1 wherein the inorganic acid metal salt is a combination of sodium chloride and potassium chloride.

14. The process of claim 7 wherein the inorganic acid metal salt comprises potassium fluoride and least the organic acid salt comprises potassium formate.

References Cited UNITED STATES PATENTS 2,551,365 5/1951 Craven 260340 3,000,861 9/1961 Brown et al. 26067 3,169,938 2/1965 Evers et a1. 26037 3,000,860 9/1961 Brown et al. 26067 FOREIGN PATENTS 905,826 9/ 1962 Great Britain.

OTHER REFERENCES Staudinger et al.: Justus Liebigs Annalen der Chemie, vol. 474, No. 198, pages 255 (September 1929). Sauterey: Comptes Rendus, 229, pages (1949).

(Other references on following page) 19 20 OTHER REFERENCES Geil: Chemical and Engineering News, vol. 43, pages Sauterey: Annales de Chemie, Ser. 12, vol. 7, pages 72 84(Aug'161965)' 23-24 (1952). Hammer et al.: Jour. App. Poly Sci., vol. No. 2, pages WILLIAM SHORT lma'y Examme" 169-178 (March, April 1959). 5 L. M. PHYNES, Assistant Examiner.

Alsup et al.: Ibid, page 191. Geil et al.: Jour. App. Physics, vol. 30 No. 10, pages 1516-17 (October 1959). 167-65; 260-340.2, 463, 471, 473, 476, 482, 488,

Geil et a1.: Growth and Perfection Crystals, John 484, 618, 619, 623, 621 Wiley & Sons, Inc., New York (1958), pages 579-585. 10

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,415, 783 December l0, l9

Silvio Bezzi et al.

It is certified that error appears in the above identified patent and that said Letters Patent are hereby corrected as shown below:

Column 3, line 26, "Beckman" should read Beckmann same column 3 TABLE 2, first column, line 1 thereof, "13'' should read l8 Column 7 line 44, "623" should read 628 Column 8, TABLE 12, second column, line 7 thereof, "0. 43" should read 0.48 line 48 18" should read l3 Column 9, line 10, "'sunpension" should read suspension same column 9, TABLE 15, line 6 thereof, "86" should read 66 Column 11, TABLE 17, third column, line 5 thereof, "34" should read 84 line 71, "35*C" should read 35 C Column 12, line 4, "CH should read CH O line 52, "searting" should read starting Column 15,

line 40, "182 g. should read 183 g. Column 18, line 54, claim reference numeral "7" should read 6 line 55 "and least the should read and the Signed and sealed this 10th day of March 1970.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. WILLIAM E. SCHUYLER, J Attesting Officer Commissioner of Patent 

1. A PROCESS FOR SYNTHESIZING HIGH MOLECULAR WEIGHT POLYOXYMETHYLENES HAVING AN ORTHORHOMBIC-CRYSTALLINE STRUCTURE WHICH COMPRISES POLYMERING AND POLYOXYMETHYLENES FROM AN AQUEOUS SOLUTION OF FORMALDEHYDE BY TOPOCHEMICAL REACTION ON AN EFFECTIVE AMOUNT OF POLYOXYMETHYLENE AS THE SOLIDE PHASE AT A TEMPERATURE RANGING FROM ABOUT 0*C. TO 60*C.; SAID AQUEOUS SOLUTION OF FORMALDEHYDE CONTAINING AT LEAST ONE INORGANIC ACID METAL SALT WHEREIN THE METAL OF THE SALT IS SELECTED FROM GROUPS I AND II OF MENDELEEFF''S PERIODIC TABLE, SAID INORGANIC SALT BEING PRESENT IN THE AQUEOUS SOLUTION AT A CONCENTRATION OF AT LEAST 6% BY WEIGHT; SAID AQUEOUS SOLUTION HAVING A PH BETWEEN 9 AND 12 AND A FORMALDEHYDE CONCENTRATION RANGING FROM THE EQUILIBRIUM CONCENTRATION TO THE SATURATION CONCENTRATION; SAID POLYMERIZATION TAKING PLACE WHILE MAINTAINING THE PH AND THE CONCENTRATION OF FORMALLDEHYDE AND INORGANIC CID SALT SUBSTANTIALLY CONSTANT. 